This invention relates to an improved protective coating composition. More particularly, it relates to a silicone resin coating composition, especially adapted for primerless adhesion to cast acrylic substrates, which when applied to these and other plastic substrates forms a protective abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylates, like Plexiglas, are also commonly and widely used glazing.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, scratch resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
Copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, discloses another abrasion-resistant coating composition. This coating composition has been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates. A particularly significant area of application for this coating is in the glazing and optical lens industry. Since these areas require a coating of high optical clarity, coatings which show flowmarks, dirt-marks or other marks which may impair visibility, are undesirable.
In copending U.S. Application Ser. No. 964,911, filed on Nov. 30, 1978, it is disclosed that the addition of a small amount of a polysiloxane polyether copolymer to the coating compositions disclosed therein eliminates the occurrence of undesirable flowmarks and the like, as well as providing other improvements in the hard abrasion resistant coating.
In many instances, however, in order to obtain good adhesion of the protective silicone resin coating composition to the substrate, a primer must first be applied to the substrate. This has been found to be especially necessary with cast acrylics, e.g., polymethylmethacrylates, like Plexiglas.
It has now been surprisingly discovered that if a solvent comprising a mixture of polar organic solvents miscible with water and alcohol and aggressive to plastic is used in the coating compositions of said U.S. Ser. No. 964,911 instead of the dilution solvents, e.g., isobutanol, suggested therein, a protective coating composition is provided which adheres to cast acrylics without the necessity of a primer.